Man-made salt caverns are used to for storage of hydrocarbons or for disposal of wastes. Such caverns may be formed during salt-mining processes (also referred to as “solution mining,” and “in situ leaching”) where the recovered salts are useful products and the salt caverns are subsequently used for hydrocarbon storage or waste disposal. Alternatively, such salt caverns may be developed specifically for hydrocarbon storage or waste disposal. In the latter situation, the mined salts will often be disposed of. Some examples of soluble salts that can be extracted by solution mining to form a salt cavern include sodium chloride, potassium chloride and sodium sulfate, among others.
To form a salt cavern according to conventional methods, well-drilling equipment is used to drill a hole from the surface to the depth of the salt formation. The portion of the well above the salt formation is supported by several concentric layers of pipe known as casing to protect non-saline water zones and to prevent collapse of the hole. To form a salt cavern, the well operator pumps non-saline water through one of the pipes. As the cavern wash water comes in contact with the salt formation, the salt dissolves until the water becomes saturated with salt. The brine then returns to the surface. Cavern space is created by the removal of salt as a salt solution which is often referred to as “brine” whether it is saturated or not.
The two types of common subsurface salt deposits are salt domes and bedded salt. Salt domes are large, generally homogeneous formations of salt that are formed when a column of salt migrates upward from a deep salt bed, passing through the overlying sediments.
Bedded salt formations occur in layers bounded on the top and bottom by impermeable formations and interspersed with non-salt sedimentary materials (such as anhydrite, shale, and dolomite) with varying levels of impermeability.
Currently, salt caverns developed for hydrocarbon and waste storage are formed by a “once through” process, wherein saline water exiting the newly formed cavern is disposed of. In this process, the water entering the cavern is always under-saturated with salt. Therefore the water will continue to dissolve salt as it enters the cavern and consequently the cavern will continue to grow. Problems associated with this process include a need for installation of pipelines to supply water and disposal wells for disposal of brine. In addition, cavern lifetimes are reduced if their diameter grows to exceed recommended limits.
U.S. Pat. No. 2,787,455 to Knappen describes a method for forming underground reservoirs for the storage of gases or liquids, particularly petroleum products. The method includes the steps of drilling a hole into a soluble-rock formation, setting casing in the hole down to a point between a substantial distance above and a substantial distance below the top of the soluble-rock formation cementing the casing, lowering inlet and outlet tubing into the hole to a depth below the casing seat, forcing non-dissolving sealing liquid through the space between the tubing and the casing to a point below the casing seat, and pumping a solvent which is immiscible with and heavier than the sealing liquid to dissolve the rock of the formation. The solvent is withdrawn and dissolving or leaching is continued until a cavity of a predetermined size is produced. These steps may be repeated for different sections of a given formation.
U.S. Pat. No. 2,994,200 to Carpenter describes a method for making underground storage caverns. The method includes the steps of circulating through an underground formation a liquid which is a solvent for the formation, forming a recess in the roof of the formation, spotting liquefied petroleum gas in the recess, and continuing the circulation of the solvent to leach out a cavern while the roof of the cavern is shielded from the solvent by the presence of the layer of liquefied petroleum gas between the solvent and the roof of the cavern.
U.S. Pat. No. 3,632,171 to French and Slezak describes a method of controlling the growth of brine wells. The method is used in solution mining to obtain brine as a product. A substantially cylindrical cavity is formed in the solution mining of soluble deposits such as salt, by the use of an oil pad to separate the solvent from the overhead soluble in the developing cavity. A well is first drilled into the salt deposit and fitted to a depth of several hundred feet into the salt deposit with a cemented casing. The lower end of the casing establishes the eventual roof of the final cavity. Within the cemented casing, and concentric to it, are hung two strings of pipe, the inner one extending below the outer pipe to a point near the bottom of the drilled well. In operation, a solvent, such as water, is pumped in the annular space between the two strings of pipe. A water-immiscible petroleum liquid fraction of lesser density than water is pumped through the annular space between the cemented casing and the outer string of pipe to form an oil pad on the surface of the water. The petroleum liquid may be used to fill the cavity to any depth desired, to form an oil pad, thus protecting the exposed salt all the way down to the interface and exposing the salt below the interface to the action of the water.
U.S. Pat. No. 4,192,555 to Willett describes a method of disposing of solid sodium chloride while selectively performing solution mining of potassium chloride from a subterranean deposit containing potassium chloride and sodium chloride. An aqueous solvent saturated with respect to sodium chloride, unsaturated with respect to potassium chloride and slurried with solid sodium chloride, is fed into the deposit having a cavity wherein there is a face on which rich and lean potassium chloride ore is exposed. Potassium chloride is thereby dissolved while sodium chloride is deposited from the solvent slurry and the resultant solution withdrawn from the cavity enriched in potassium chloride.
U.S. Pat. No. 4,249,833 to Talley describes a method for depressurizing a leached salt cavern which includes an upper hydrocarbon blanket to protect the roof of a cavern. Depressurization of a cavern is required when measurements of the cavern, such as sonar measurements are required. To achieve depressurization, the following steps are carried out: (1) petroleum liquid is injected into the annulus between the leach strings (including an input string for non-saline water and an output string for brine), to the same depth as the petroleum liquid in the outer annulus, the long leach string, which is the central leach string, is open; and the displaced aqueous liquid moves up and out the central string; (2) a packer plug is then inserted in the central string to about this same level; and the aqueous liquid above the packer is replaced with petroleum liquid; and (3) the replacement of the aqueous liquid above the packer plug with petroleum liquid of lower density means that there is now a higher pressure on the underside of the plug than on the upper side; and this pressure is relieved through the plug through tubing to the surface, whereupon the packer plug is retrieved, leaving the well equalized.
U.S. Pat. No. 5,004,298 to Boulanger and Rousseau describes a method for preparing large cavities for abandonment. When an underground quarry or mine has been worked out, there remain underground voids which, in the absence of suitable support measures being taken, run the risk of collapsing and giving rise to ground subsidence which is damaging to surface infrastructure. Therefore, prior to ceasing to monitor cavities, it is appropriate to implement means suitable for avoiding any subsequent disturbance. The method includes the steps of injecting a mixture (which may include waste materials) with density greater than that of the brine, and the mixture being capable of setting, until the cavity is completely filled with the mixture and the brine is displaced; maintaining communication between the outside and the filled cavity during a waiting period whose length is exclusively determined by the time required for the injected mixture to set and for the rock salt to creep into and close shrinkage voids that develop in the set mixture, without regard to the establishment of thermal equilibrium in and surrounding the cavity; and thereafter sealing the cavity.
U.S. Pat. No. 7,097,386 to Maduell et al. describes a method for simultaneously developing caverns while depositing wastes or other materials in them. The method comprises drilling a well into a naturally occurring salt formation and initiating the development of a salt cavern by means of solution mining techniques so as to mine the formation of salt with water (seawater or fresh water). When the initial development of the salt cavern in this fashion has been carried out to an extent sufficient to accommodate the injection of a prescribed amount of such wastes or other materials into the cavern, injection of the wastes or other materials through the well is started while continuing to develop the cavern by solution mining techniques. The injection of the wastes or other materials may be carried out continuously (into the constant flow of solution mining water), or intermittently (at time intervals between successive injections of solution mining water). The proportion and rates of wastes or other materials and solution mining water injected into the well are monitored and regulated so that cavern development continues in a manner and at a rate that allows the cavern to reach an intended prescribed size while the wastes or other materials are injected and deposited into the cavern.
U.S. Pat. No. 7,156,579 to Castle et al. describes a process for manufacturing underground caverns suitable for storage of large volumes of gaseous or liquid materials. The method is an acid dissolution process that can be utilized to form caverns in carbonate rock formations. The method can also be utilized to form calcium chloride as a by-product of the cavern formation process. The method includes the steps of drilling a first well into a subterranean formation comprising carbonate rock; pumping a low viscosity aqueous acid solution through the first well; ejecting the aqueous acid solution from the first well to contact carbonate rock of the formation; reacting the aqueous acid with the carbonate rock to form reaction products comprising salt in an aqueous solution and carbonic acid, wherein the carbonic acid is in equilibrium with carbon dioxide; and removing the reaction products from the subterranean formation to form a cavern defined by the remaining subterranean formation.
There remains a need for improvements in development and maintenance of salt caverns for storage and disposal of materials produced during natural resource extraction.